They say that people tend to become less wary of things when they have a prior experience with such a thing. It is indeed true and the reasons are numerous.
The human mind is one lazy fellow by nature, if working harder than normal was not necessary even if it was in a quite threatening situation, then it will automatically become less cautious. That phenomenon is the main reason for what we call 'mistakes' that humans make.
But this thing didn't seem to work in Flora's case, or to be more precise, she never allowed for that to happen. Even though she was tired physically and most importantly mentally, she never relaxed. This tenseness was exhausting her even more, but it also helped her in reducing the chances of her getting attacked by the unknown things lurking around.